Monolithic or monolith forming refractories are special mixes or blends of dry granular or stiffly plastic refractory materials, with which virtually joint free linings are formed. They embrace a wide range of mineral compositions and vary greatly in their physical and chemical properties. In various types of furnaces, monolithic refractories are used to advantage over brick construction. By their use, front installation can be made, avoiding delays for the manufacture of special brick shapes. They frequently eliminate difficult brick laying tasks which may be accompanied by weakness in construction. They are of major importance in the maintenance of furnaces. Substantial repairs can be made with a minimum loss of time and in some instances even during operations.
Gunning mixes consist of granular refractory materials specifically prepared for application with air placement guns. Generally, there are two types of gun mixes based on their industrial uses; namely, those of high density and strength characterized by resistance to heat, chemical attack and mechanical abuse and those of lower density and strength designed primarily for insulating purposes. The mixes are sprayed from the guns or "gunned" at high velocity and pressure, forming homogeneous and densely compacted linings essentially free from lamination cracks.
Basically there are two types of air placement guns, the dry and the wet. With the dry gun, the dry mix becomes uniformly moistened with water while it is passing through the nozzle during the gunning operation. For use with the wet gun, the material is mixed with water within the chamber of the gun before it is fed to the nozzle. Gunning mixes are generally furnished in dry form. Some have air setting properties; others are heat setting.
One of the most serious problems in gunning a refractory is the high losses due to refractory material bouncing back off a wall or the site upon which it is being emplaced. The loss is termed "rebound" in the art. Rebound loss which has been as high as 50% and 60%, is not only undesirable from an economic standpoint but also results in significant alteration of the monolithic chemistry which normally decreases refractoriness.
Accordingly, it is an object of the present invention to provide improved methods of in situ fabrication of monolithic refractory linings by gunning.
Another object of the invention is to provide improved methods for gunning refractory material with a relatively low rebound loss.
Still another object of the invention is to provide improved methods of gunning refractory materials wherein the beneficial characteristics of the material are substantially similar to materials of the same chemical composition emplaced by other methods.
In accordance with the present invention, there is provided a method of gunning a refractory material on its situs of use, with a minimum of rebound, to form a refractory monolith. Suitable size graded, non-basic refractory material is prepared and mixed with a calcium aluminate cement. To this refractory mixture is added from about 0.5 to 25%, based upon the weight of the cement, of calcium chloride hydrate. The resulting batch is tempered with sufficient aqueous material to allow gunning and the resulting mixture is gunned on the situs of use. The use of calcium chloride hydrate (CaC1.sub.2 . 2H.sub.2 0 in gunning mixes has been found to reduce rebound losses as much as 75%.
In a preferred embodiment, the refractory aggregate contains Al.sub.2 O.sub.3 and SiO.sub.2 as major components and the calcium chloride hydrate is present in amounts between about 0.5 and 15%, based on the weight of the cement.
The non-basic refractory aggregate may consist of calcined alumina, tabular alumina, calcined koalin, calcined bauxite, kyanite, pyrophyllite, calcined clays, fire clay, ball clay, combinations thereof and calcium aluminate cement. The calcium aluminate cement may be any of the commercially available ones ranging from high to low purity. An exemplary chemical analysis of the usable calcium aluminate cement is as follows: 0.1% SiO.sub.2, 79% Al.sub.2 0.sub.3, 0.3% Fe0 and Fe.sub.2 0.sub.3, 18% CaO, 0.4% MgO and 0.5% total alkalies. Another usable cement typically analyzes: 9.0% SiO.sub.2, 39% Al.sub.2 0.sub.3, 12% Fe.sub.2 0.sub.3, 39% CaO and 1% MgO.